There are three major mammalian isozymes of pyruvate kinase (ATP: pyruvate phosphotransferase, EC 2.7.1.40). We have isolated two of them, the skeletal muscle and liver isozymes, from bovine tissues and have hybridized the enzymes in vitro. The isolated hybrids, designated L3M, L2M2, and LM3, consist of subunits taken in part from an allosteric (L4 or liver) and nonallosteric (M4 or skeletal muscle) parental enzyme. We propose to continue with the characterization of the hybrids in order to determine the contribution to kinetic activity of each type of parental subunit. In addition to the above, we propose to isolate and characterize the third mammalian isozyme, type M2.